The present invention relates to gas flow measurement, and more specifically to a differential pressure flow meter that is used to determine the flow rate of a gas.
In hospitals, both in intensive care and during operations, it is necessary to perform human respiration artificially using a mechanical ventilator. The unhindered flow of gases to and from the lungs is naturally of vital importance. The functioning of the gas passages can be monitored both by measuring the contents of the inspired gases and by measuring the flows and pressure. Many different technologies have been applied to create a flow meter that meets the requirements of the critical care and operating room environment. Among the flow measurement approaches that have been employed are:
1) Differential pressure--measuring the pressure drop or differential across a resistance to flow.
2) Spinning vane--counting the revolutions of a vane placed in the flow path.
3) Hot wire anemometer--measuring the cooling of a heated wire due to airflow passing around the wire.
4) Ultrasonic Doppler--measuring the frequency shift of an ultrasonic beam as it passes through the flowing gas.
5) Vortex shedding--counting the number of vortices that are shed as the gas flows past a strut placed in the flow stream.
6) Time of flight--measuring the time of an impulse of sound or heat created upstream to a sensor placed downstream
At the present time, the most commonly employed device for gas flow measurement in critical care and operating room environments is the differential pressure flow meter. Because the relationship between flow and the pressure drop across a restriction or other resistance to flow is dependent upon the design of the resistance, many different resistance configurations have been proposed. The goal of all these configurations is to achieve a linear relationship between flow and pressure differential. As is well known, flow in a tube can be laminar or turbulent. In the laminar case, the pressure difference across a flow restricting body placed in the path of flow is directly proportional to the rate of flow. For turbulent flow, the pressure difference is a function of the square root of the rate of flow.
In some prior art differential pressure flow meters (commonly termed pneumotachs), the flow restriction has been designed to create a laminar flow that results in a linear relationship between flow and differential pressure. For gas flow rates up to 3 liters/sec, the flow restrictor can be a porous plastic or metal filter 10 with pore sizes on the order of 25 micrometers, as shown in FIGS. 1a and 1b. Another type of flow restrictor includes a bundle of small diameter tubes 12 with appropriate length and diameter to achieve the desired pressure drop, as shown in FIG. 2. In each case, the flow restrictor is designed in an attempt to ensure laminar flow and a linear response to flow.
Therefore, it is an object of the present invention to eliminate the above-identified problems associated with the prior art differential pressure flow meters. Additionally, it is an object of the invention to provide a flow sensing device including a flow restrictor that promotes laminar flow of the gas.